fix(android): preserve cursor position when centered TextInput is cleared

[HarshitMadhav]

Summary:

On Android, clearing a TextInput with textAlign: "center" caused the cursor to jump to the right edge of the field instead of staying centered. iOS was not affected. See #55457 for a video reproduction.

The cause is setText(null) in ReactEditText.maybeSetText. On AppCompatEditText, that call replaces the underlying Editable buffer with a freshly constructed one, and the new buffer doesn't carry forward the gravity-based caret positioning that Gravity.CENTER_HORIZONTAL relies on. The cursor then snaps to the layout's natural edge — the right side for LTR centered text — until the user taps the field again.

This PR fixes the issue by clearing the text in place on the existing Editable rather than swapping the buffer:

• BaseInputConnection.removeComposingSpans(currentText) — preserves the original "some buggy keyboards don't clear the composing text on their own" intent that motivated the original setText(null).
• currentText.replace(0, currentText.length, "") — empties the buffer without re-creating it.

This mirrors the approach already used a few lines below for the non-empty branch (checkNotNull(text).replace(0, length(), spannableStringBuilder)), so the empty-text path is now consistent with the non-empty path. Gravity is preserved on the same Editable, so the cursor stays centered (or right-aligned for RTL) after clearing.

Fixes #55457.

Changelog:

[ANDROID] [FIXED] - Cursor no longer jumps to the right edge when a centered TextInput is cleared

Test Plan:

Added a Robolectric regression test ReactTextInputPropertyTest.testClearingTextPreservesEditableBufferAndGravity that:

1. Applies textAlign: "center" and asserts gravity is Gravity.CENTER_HORIZONTAL.
2. Sets the text to "hello" via ReactTextUpdate and captures view.editableText as bufferBeforeClear.
3. Clears the text via another ReactTextUpdate with an empty string.
4. Asserts view.editableText is the same instance as bufferBeforeClear — this is the discriminating behavioral guarantee of the fix vs. setText(null), which would replace the buffer.
5. Asserts the text is empty, gravity is still centered, and selectionStart/selectionEnd are both 0.

The test is a true regression test:

• Passes against the fix.
• Fails against the original text = null code with Expecting actual: hello and actual: "" to refer to the same object.

Ran:

./gradlew :packages:react-native:ReactAndroid:testDebugUnitTest \
  --tests "com.facebook.react.views.textinput.ReactTextInputPropertyTest"

Result:

BUILD SUCCESSFUL
tests=23 skipped=0 failures=0 errors=0

All 22 pre-existing tests in the class continue to pass, so no behavior is regressed by the fix.