Apply correct bottom padding to layouts

The returned descender value for BoringLayout and StaticLayout should
be equal to the font's "bottom" metric in the includePadding case.
Previously, the calculation incorrectly included an addition
mBottomPadding value in some cases (which was an attempt to work
around the case where maxLines was set on TextView but not the
corresponding StaticLayout, no longer the case).

Bug: 27901763
Change-Id: I008c5974b979087725a9bb9104e4771b0caac01c