BootAnimation: remove superfluous null check for `mAnimation`

The null check added in I603da080b1732a436a10dbaca6f2be9f158124dc makes
our static analyzer suspect that the call to
`playAnimation(*mAnimation)` dereferences null. The beginning of this
function establishes a non-null value for `mAnimation`, which is used
unconditionally throughout the function. Hence, this condition seems
unnecessary.

Bug: None
Test: TreeHugger
Change-Id: I0575df17df7f7e3018d99cafd9dcfcd3c10e5fbc