Frameworks/base: Refactor android.os.Debug

Refactor some static fields into a holder. This allows to statically
initialize the class in the boot image. It also has some minimal
positive impact on the class size, a net benefit when tracing is
not used.

Bug: 27248115
Change-Id: I0cdfb6acf916ad8d9a7d82bba32929dba15d4200