Build: add hidden method to parse a major.minor String to int

Add a method to parse a string representation of a major.minor SDK
version (e.g. "43.21") into an int, encoded in the same way as
SDK_INT_FULL. This will be used by the package parser when reading
manifest values that include minor versions.

Bug: 377689343
Test: atest 'FrameworksCoreTests:android.os.BuildTest'
Flag: EXEMPT unused flag, call sites will be flagged when added
Change-Id: Ice31da5bbb6de5d2a8938c606c56738c8943f440