Expose Handler methods for mainlining VCN

Expose the following three methods as a prerequisite
for moving VCN to mainline.
- #hasMessagesOrCallbacks()
- #removeCallbacksAndEqualMessages(Object)
- #removeEqualMessages(int, Object)

Bug: 366598445
Test: atest CtsOsTestCases:HandlerTest
Flag: android.os.mainline_vcn_platform_api
Change-Id: If16bbcdd5aba7438397f50d82b2ffd1555b2dddf
Merged-In: If16bbcdd5aba7438397f50d82b2ffd1555b2dddf